Root bone as pelvis vs ground?
Having the ground bone as part of the same armature makes it much easier to animate as you can just stay in pose mode for the one armature the whole time. Furthermore actions are designed to be applied to single objects so it helps there as well.

The only reason why you might not have a root ground bone is if you are planning on exporting the rig to another piece of software with different expectations (e.g. matching a default skeleton in a game engine that has the root at the pelvis). In this case parenting to an empty isn't going to be that much help either though as the program likely won't know what to do with that either.